In a known apparatus of this kind (JP-A-52 2 8 526) the upper portion of a nozzle device is controlled in response to signals of a thickness sensor by means of a vertical lifting device for compensating thickness variations of the products so as to keep constant the distance between the nozzle openings of the upper nozzle device and the surface of the semifinished product. When hot-rolling slabs and steel strips, the forging scales have to be removed before performing the rolling process to obtain a good surface. It is known to carry out such a descaling process by means of spraying highly pressurized water at pressures of about 1000 bar onto the surface to be descaled. A known descaling apparatus for descaling by means of highly pressurized water is described in U.S. Pat. No. 5,502,881. In the nozzle device described in this patent, a plurality of rotating nozzle heads each having four nozzles distributed around the outer circumference thereof for spraying the highly pressurized water, are attached to a girder fixedly mounted and disposed transversely to the moving direction of the strip conveyed underneath and at such a distance that bulgings and unevenness of the strip conveyed past the nozzle device do not lead to a collision. This distance is relatively large in the known apparatus. This leads to a reduction of the spray pressure of the highly pressurized water at the workpiece. A sufficiently high spraying pressure can only be achieved by increasing the water pressure or the amount of water, which results in a great power consumption. A reduction of the distance of the nozzle device to the strip surface is not possible because of the above described risk of a collision.